线程管理（一）AsyncTask与Thread的差别
AsycTask，做Android的应该都是熟悉的不能再熟悉了，当我们在进行耗时操作的时候，就可以使用这个了
但是有的人就在想，Thread不是也可以开一个子线程进行耗时操作吗？那这两个有什么样的区别呢？
那么一起来看下AsycTask源码啦
public abstract class AsyncTask {
private static final String LOG_TAG = "AsyncTask";
//获取cpu可用处理器的数量，线程的数量为cpu*2+1比较好
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE = 1;
//这里有个创建线程的方法，说明AsyncTask里面是含有Thread的
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//创建一个Linked的队列，先进先出
private static final BlockingQueue sPoolWorkQueue =
new LinkedBlockingQueue(128);
//创建线程池啦
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
/**
* An {@link Executor} that executes tasks one at a time in serial
* order. This serialization is global to a particular process.
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static InternalHandler sHandler;
private final WorkerRunnable mWorker;
private final FutureTask mFuture;
private volatile Status mStatus = Status.PENDING;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
//创建一个空数组双端队列容纳16个元素的初始容量
private static class SerialExecutor implements Executor {
final ArrayDeque mTasks = new ArrayDeque();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
//从队列中取出最先的并执行
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
}
//从这里开始使用Handler机制，用来跟主线程进行交互
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
public AsyncTask() {
mWorker = new WorkerRunnable() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//我们实现的doInBackground方法，就是在这里调用的
return postResult(doInBackground(mParams));
}
};
//使用来管理线程的
mFuture = new FutureTask(mWorker) {
//这里就是耗时操作完成了
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occured while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
//这里就是向主线程发送消息
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult(this, result));
message.sendToTarget();
return result;
}
public final Status getStatus() {
return mStatus;
}
protected abstract Result doInBackground(Params... params);
protected void onPreExecute() {
}
@SuppressWarnings({"UnusedDeclaration"})
protected void onPostExecute(Result result) {
}
@SuppressWarnings({"UnusedDeclaration"})
protected void onProgressUpdate(Progress... values) {
}
@SuppressWarnings({"UnusedParameters"})
protected void onCancelled(Result result) {
onCancelled();
}
protected void onCancelled() {
}
public final boolean isCancelled() {
return mCancelled.get();
}
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return The computed result.
*
* @throws CancellationException If the computation was cancelled.
* @throws ExecutionException If the computation threw an exception.
* @throws InterruptedException If the current thread was interrupted
* while waiting.
*/
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
public final AsyncTask execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
public final AsyncTask executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
//我们调用的new AsycTask（。。）.execute就是调用线程池来执行异步任务
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult 简单的捋一下，就是通过线程池创建线程执行异步任务，然后通过handler机制，发信息给主线程，让主线程知道现在是出于什么样的状态
区别：1。AsycTask跟Thread的区别就是AsycTask它能管理的线程数量是有限的！
MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
2。AsycTask是使用了Handler机制能跟主线程进行交互，而Thread只是异步操作
那么，如果我们只是进行异步操作不跟主线程进行交互，还需要使用AscyTask吗？
如果我们需要用到很多的异步操作，能使用AsycTask吗？
当然就是不能拉，下一节介绍，我们自己建立一个线程管理器